Using different work function nanocrystal materials to improve the retention characteristics of nonvolatile memory devices
Introduction
Since Kahng and Sze [1] proposed the first floating gate semiconductor nonvolatile memory (NVM) device in 1967, the research attention of NVM has been considered important from the end of the 20th century to date, and it has been successful currently. In floating gate memory devices, charges or data are stored in the floating gate and are retained when power is removed. Along with the development of NVM in the nano-scale field, using nanocrystals (NCs) as a floating gate has attracted great interest and research attention and has exhibited great potential of replacing conventional floating gates in NVM devices [2], [3], [4], [5]. In NC floating gate memory devices, NCs such as semiconductor [6], [7], metal [8], [9], compound [10] or hetero-NC [11] are embedded between the tunneling dielectric layer and the control dielectric layer as a charge storage layer. It is well known that NC memory has several advantages over the conventional floating gate memory, such as lower leak current, better endurance and retention performance and so on. This is because NC films include discrete NCs, and each NC acts as a small floating gate; therefore, even if a charge leakage path exists between the small floating gate and the substrate or the control gate, only a small part of charges stored in the NC film is lost. To date, NC devices are considered as one of the promising candidates for future memory technological applications.
In this letter, different work function NC materials are used as the floating gate in NVM devices. The impact of work function on the retention characteristic is investigated and the physical mechanisms of NC memory with different floating gates are discussed at the same time.
Section snippets
Experiment
The typical floating gate NC NVM structure is shown in Fig. 1. NC memory has an NC film as the floating gate/charge retention layer between the tunneling dielectric layer and the control dielectric layer. The fabrication process of NVM devices with different work function NC materials as the floating gate can be generalized as follows: after chemically cleaning the p-type (1 0 0) silicon wafer, the tunneling dielectric layer SiO2 with a thickness of 5 nm is formed by electron beam evaporation;
Result and discussion
The charge retention characteristics of the three samples with different work function materials as the floating gate have been measured for the purpose of demonstrating the impacts of work function on the retention time. Fig. 2 shows the energy band diagrams of NC NVM devices with different floating gates, and the work functions of different floating gates are shown in Table 1. However, as the work function of Au (5.0 eV) is larger than that of W (4.6 eV) and Si (4.05 eV), a deeper potential well
Conclusion
Floating gate NC NVM devices with different work function materials as floating gates were fabricated, and the effect of work function on retention performances was investigated. The results obtained from this research show that NVM devices utilizing Au-NC as floating gates show good retention characteristics as compared to W-NC and Si-NC memories. It is found that the larger work function NC plays an important role in NVM and has achieved better retention characteristics. Therefore, studying
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